No more than 33,000 years old – that’s the assessment of astrophysicist Dr. Jenny Feige and her colleagues at TU Berlin’s Zentrum für Astronomie und Astrophysik (Center for Astronomy and Astrophysics) of the 19 atoms in the radioactive isotope iron-60 that they found in sediments from the deep sea. Since this type of iron does not occur naturally on Earth, it must have dropped down from outer space, probably originating from a stellar explosion, a so-called supernova.
Dr. Feige is part of an international team of scientists based at locations in Canberra (Australia), Vienna, Berlin and Dresden and headed by Professor Dr. Anton Wallner of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). It is now some years since his team succeeded in finding radioactive stardust, enabling them to prove that several supernovas must have occurred in relatively close proximity to our solar system, both around 2.6 million and six to seven million years ago. The stardust under investigation originates from sediments found 4,000 meters below sea level near the south-western tip of Australia.
“These sediments are like an archive that tells us the history of our planet and that of the universe,” explains Feige, “and they give us completely new insights into the structure and development of our Milky Way.” Then, as now, she is responsible for carrying out the most crucial parts of the investigations, including those involving the HZDR’s accelerator mass spectrometer at the large-scale DREAMS facility. Providing proof for the hypothesis is extremely difficult, because the extraterrestrial iron must first be separated from the terrestrial iron in complex chemical processes. Did a supernova occur near the earth a few millennia ago in the human epoch?
“Our discovery only implies in the first instance that iron-60 from supernovas reached us in the more recent past, certainly less than a few million years ago,” says Feige. And it proves that for at least 33,000 years our Earth has been moving through a dense cloud of gas and dust known as the local interstellar cloud, which could also have an influence on our climate and the radiation to which the Earth is exposed. Had the cloud been formed from a supernova during the last few million years, it would contain more iron-60. Sometimes, however, such clouds spread out in space over millions of years.
Anton Wallner, head of the research project, explains: “It’s also possible that the iron-60 that’s trapped in dust particles within the interstellar medium could have been reflected several times – ‘pushed around’ so to speak – and could therefore originate from older supernova explosions. In that case we would be measuring a kind of echo of these cosmic eruptions.” Only further research can provide the answers.
The study was published in the journal "Proceedings of the National Academy of Sciences" (PNAS) DOI:10.1073/pnas.1916769117.